pressure equilibrium of two gases, using the difference the thermal conductivity of the two gases, and Particularly to a circuit arrangement that provides appropriate control of the temperature of a heated filament sensor. In addition, the arrangement is free of the influence of changes in ambient temperature and humidity.
The need for accuracy in determining very small amounts of trace gases is discussed in U.S. patent application Ser. Nos. 763,290 and 825,344 by Mark Warchol et al and Warchol, respectively, and also in U.S. Pat. No. 4,454,748 to Terai et al. In the first Warchol et al application, the "Telegas" process and apparatus of U.S. Pat. No. 2,861,450 to Ransley is combined with a device capable of computing the percent of gas content in a molten supply of metal from readings of the thermal conductivity of the content gas and of a carrier gas by a catharometer and from the readings of temperature of the molten supply, all of which is then modified by a conversion factor of the alloy of the molten supply, which is known. Before the Warchol et al disclosure, all of the above readings, measurements, and conversions were done by using temperature charts and alloy tables. The reading of the charts and the arithmetic involved in making the changes often resulted in errors in determining the amount of gas contained in the molten metal. By combining the entire process in the operations of the computing device, these errors were eliminated.
The second of the above Warchol applications eliminates the reference cell employed in the catharometer of the first application, as well as in the Ransley patent, and, inter alia, uses a constant current source in series with the remaining hot-wire sensor. In this manner, the voltage drop across the sensor changes only in response to the precise amount of gas content reaching the sensor. In addition, the elimination of the reference cell eliminated the cumbersome task of providing identical hot-wire sensors needed for accuracy in such systems. As explained in the Warchol application, such sensors are often hand wound, which is tedious and time consuming, and ultimately does not guarantee precise matching of the sensors.
The above Terai et al patent improves the accuracy of the Telegas measurement by isolating the hot-wire sensors from changes in ambient temperature This is effected by disposing the sensors in a housing and then evacuating the interior of the housing. As can be appreciated, evacuating apparatus adds cost and bulk to the instrument, where compactness is needed, as such instruments are often portable devices used on line in casting operations, and cost reductions, as opposed to increases, are sought to meet competition.
In the Ransley system, changing ambient required several readings of partial gas pressures before a stable reading was obtained to provide reasonably accurate indications of the partial pressures. (Partial pressure is the equilibrium Pressure of gas molecules located at a free surface in a body of molten metal in which the gas is dissolved. If the solubility of the gas at a given pressure [e.g., 760 mm of mercury] is known, then a given gas content in the molten metal will give rise to an internal or equilibrium pressure.)
Further, because the reference cell in Ransley was open to the atmosphere, the instrument was subject to error because of changes in humidity. Moisture affects directly the thermal conductivity of the atmosphere such that the hot wire in the reference cell recorded changes in the moisture content of the atmosphere which introduced reading errors.